THE OPTICAL COMPANION TO THE INTERMEDIATE-MASS MILLISECOND PULSAR J1439–5501 IN THE GALACTIC FIELD

We present the identification of the companion star to the intermediate-mass binary pulsar (PSR) J1439-5501 obtained by means of ground-based deep images in the B, V, and I bands, acquired with FORS2 mounted at the European Southern Observatory (ESO)-Very Large Telescope (VLT). The companion is a massive white dwarf (WD) with B = 23.57 ± 0.02, V = 23.21 ± 0.01, and I = 22.96 ± 0.01, located at only ∼0.″05 from the pulsar radio position. Comparing the WD location in the (B, B-V) and (V, V-I) color-magnitude diagrams with theoretical cooling sequences, we derived a range of plausible combinations of companion masses (1 M ≲ M COM ≲ 1.3 M ⊙), distances (d ≲ 1200 pc), radii (≲ 7.810-3 R ⊙), and temperatures (). From the PSR mass function and the estimated mass range we also constrained the inclination angle i ≳ 55° and the pulsar mass (M PSR ≲ 2.2 M ⊙). The comparison between the WD cooling age and the spin-down age suggests that the latter is overestimated by a factor of about 10